1. Field of the Invention
The present invention relates generally to aircraft automatic flight control systems and more specifically to speed control and deceleration to a lower speed during the descent portion of flight from a cruising altitude. The present invention may be incorporated in an aircraft performance management system (PMS) which is fully coupled to the aircraft automatic pilot system.
2. Description of the Prior Art
Most commercial transport aircraft, general aviation aircraft and military aircraft are equipped with an automatic flight control system and many also include a separate performance management system for controlling the vertical flight path profile in the most efficient and economical manner, the PMS preferably being fully coupled to the aircraft's autopilot system. Such systems generally provide the human pilot with the capability of altering the flight path of the aircraft to achieve and maintain a desired speed, measured either in knots or Mach number.
During a descent from a cruising altitude, the pilot may elect to decrease the speed of the aircraft through a manually-entered speed command or the pilot may be mandated by Air Traffic Control (ATC) to decelerate to a specified speed and a specified altitude in order to maintain air traffic separation and the like. In addition, in the United States, the Federal Aviation Administration (FAA) requires that an aircraft's speed be no greater than 250 knots indicated airspeed at altitudes less than 10,000 feet. Thus the human pilot is generally required to decelerate the aircraft in a number of successive steps as the aircraft descends from a cruise altitude toward the airport at which a landing is intended.
In the prior art, these decelerations were generally accomplished by increasing the pitch attitude of the aircraft in a manner proportional to the difference between the new speed command and the actual speed of the aircraft, or speed error. While this scheme will decelerate the aircraft to the desired Mach or airspeed, there is no assurance this will be done in a timely or optimal manner or that any altitude restrictions imposed by either Air Traffic Control or the Federal Aviation Administration will be met. Another of the prior art systems is described in present assignee's U.S. Pat. No. 4,021,009 entitled "Vertical Path Control for Aircraft Air Navigation Systems" dated May 3, 1977. In this system, the deceleration of the aircraft in the descent phase of flight is accomplished by the computation of a flight path angle used to manipulate the actual vertical path of the aircraft for the achievement of a specified waypoint. Such a system suffers from relatively long deceleration times beginning at relatively high altitudes above the desired point where the newly commanded speed is to be achieved. Hence, the deceleration is neither optimal nor timely, particularly in the aircraft terminal area where precise and rapid speed control is necessary owing to constraints imposed by air traffic control. An additional prior art system is described in present assignee's U.S. Pat. No. 3,196,688, entitled "VTOL Craft Deceleration Control System", dated Nov. 4, 1975. This system, as the title emphasizes, is specifically designed for VTOL (Vertical Takeoff and Landing) aircraft and is specifically concerned with the deceleration of the aircraft as it approaches its final touchdown point. Hence, it does not address the problem of the deceleration of aircraft at high altitudes nor the achievement of the required 250 knot speed restriction at 10,000 feet of altitude. Furthermore, the method of decelerating the aircraft relies on the prescheduling of the time rate of change of the newly commanded speed, i.e., mach or airspeed rate. Such a system necessarily presupposes that the actual airspeed of the aircraft will follow exactly the predetermined rate. In fact, this is often not the case because of the effects of atmospheric turbulence, system tolerances and errors, and the like.
The present invention overcomes the shortcomings of the prior art by computing an altitude based upon the existing descent rate, or altitude rate of the aircraft, which computation is used to tend to level the aircraft with the engine (s) at idle thrust until the difference between the desired speed and the actual speed of the aircraft is within a predetermined amount, at which time conventional speed control proportional to speed error is resumed.